[x] improve error messages while loading config
[x] improve error messages while loading config

Encountered this because I mistakenly left a conflict marker in x.toml
while resolving merge conflicts. The with_context() makes it easier to
understand why the error is happening.

aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core